One of the most intriguing questions in biology is to explain how different cell types arise from a common precursor, the fertilized egg. Tissue interactions are necessary for specifying the fates of cells in the ectoderm of the chicken embryo as the developmental potential of a cell becomes restricted to a single differentiated phenotype. One of the classic systems for studying tissue interactions is lens induction by the optic vesicle. By 36 hours of development presumptive lens ectoderm cells have received sufficient information that if they are removed from the embryo and grown in culture, differentation into the lens phenotype will proceed. However, there are other regions of head ectoderm, never in contact with the optic vesicle, that will form lenses in vitro. In order to identify the early tissue interactions that establish a region with lens-forming potential, objective criteria are needed to distinguish between presumptive lens ectoderm and non-lens ectoderm at this time. As a first step in understanding how tissue interactions specify the fates of cells, we are testing the hypothesis that changes in the synthesis of ectodermal glycoproteins occur as a result of the interactions that determine which cells become lens or skin (dorsal trunk region) in the chick embryo. Recent experiments labeling tissue fragments of 36 hour embryos with 3H-sugars have revealed differences in glycoproteins synthesized by presumptive lens ectoderm and trunk ectoderm and we propose to expand our analysis of these molecules by one and two dimensional polyacrylamide gel electrophoresis of labeled tissue fragments. With these markers in hand it will be possible to determine at what stage regional differences appear in the ectoderm and to define the spatial boundaries of these differences. Finally, it should be possible to identify the other tissues that serve as early lens inductors. The long term goal of this research is to develop antibodies to these glycoproteins to precisely study temporal and spatial synthesis of these markers and to attempt to determine whether these molecules play a role in lens formation.